There have been conventionally known various signal processing devices to perform signal processing on an inputted signal. For example, there is an audio signal processing device such as a mixer that inputs an audio signal and performs various processes such as mixing on the inputted signal to output the processed signal. With regard to such a signal processing device, there is one provided with a function in which at an input stage of a signal, gain control is performed and thereby the level of the signal is adjusted to fall within a range proper for performing signal processing. In the mixer, a head amplifier is provided at an input stage of an input channel (ch) for processing an audio signal (see the description of a head amplifier in NPL1). The head amplifier is an amplifier for adjusting a level of an input signal to fall within a range proper for performing signal processing (for example, an equalizer, a compressor, or the like) in an input channel. A user of the mixer adjusts the gain to be a possible higher level in a range where a level indicated by a meter does not become OVER.